1. Field of the Invention
This invention relates to a device and method for determining the relative amount of an oxygen containing gas (hereafter called "measurement gas") in a gas mixture comprising at least one other oxygen containing gas, e.g., a humidity sensor for determining the amount of water vapor in air.
2. Discussion of the Related Art
Humidity sensors are of considerable usefulness in a variety of applications. For example, humidity sensors have been used for environmental control in computer rooms, commercial aircraft and automobiles. They are also useful in automotive and aircraft systems for carburetion control. A humidity sensitive device is typically structured such that opposing electrodes are formed on an insulating substrate and a humidity sensitive film is formed on the surface of the insulating substrate and at least between the opposing electrodes. The humidity sensitive film comprises a material exhibiting a predetermined humidity-resistance value characteristic. Accordingly, the inherent resistance value of the humidity sensitive film is variable as a function of an ambient humidity and as a result, a humidity condition can be determined in terms of a resistance value of the humidity sensitive device. Exemplary of the humidity sensitive film are organic materials such as cellulose and metal oxides such as aluminum oxide. Exemplary of devices of this type are those disclosed in U.S. Pat. Nos. 4,288,775; 4,456,902; and 4,497,701. Unfortunately, however, prior art humidity sensors are less than desirable for automotive use for one or more of the following reasons: poor reliability, lack of durability, complexity and cost.
In the last 20 years, several different types of sensors based on O.sub.2 -pumping ZrO.sub.2 cells have been developed. Such oxygen-pumping is based on the fact that if a current is Passed through an oxygen ion-conducting electrolyte (e.g., zirconia), oxygen is transferred (pumped) from one side of the electrolyte to the other. The oxygen may come from oxygen gas or gaseous compounds containing oxygen, e.g., H.sub.2 O, that are disassociated at the electrolyte. Such sensors have the common characteristic that their signal output is linearly proportional to the partial pressure of the oxygen transferred. As discussed, e.g., in "High Temperature Oxygen Sensors Based on Electrochemical Oxygen Pumping", E. M. Logothetis and R. E. Hetrick, Fundamentals and Applications of Chemical Sensors, 1986, American Chemical Society, the sensors may be of the single or double cell type.
In single-cell sensors, the same ZrO.sub.2 cell is used for both oxygen pumping and sensing. In double-cell sensors, different ZrO.sub.2 cells are used for oxygen pumping and sensing. U.S. Pat. No. 4,547,281 to Wang is directed to a single cell device capable of sensing the concentration of oxygen in a volume. Double cell sensors capable of sensing the concentration of oxygen in a volume are disclosed, e.g., in U.S. Pat. Nos. 4,272,329, 4,272,330, and 4,272,331 to Hetrick and Hetrick et al; 4,498,968 to Yamada et al; 4,645,572 to Nishizawa et al; and 4,487,680 to Logothetis et al. The Hetrick, Hetrick et al and Logothetis et al patents are commonly assigned with this invention. In general, in these two cell devices, one cell is used to pump a certain (variable) amount of O.sub.2 out of a cavity formed between the cells and the second cell (the sensor cell) is used to measure the reduced partial pressure of O.sub.2 inside the cavity. as described in the patent to Logothetis et al, the structure of that device has been modified to eliminate the cavity and employs only three electrodes, instead of the common four, but operates analogously to those of the '329, '330 and '331 patents discussed above.
Oxygen sensors based on oxygen-pumping are able to measure the amount of O.sub.2 in a mixture comprising another oxygen containing gas such as H.sub.2 O, because oxygen is pumped out at a voltage lower than that required to disassociate the other oxygen containing gas (H.sub.2 O). Such prior devices, however, were not able to determine the percentage of H.sub.2 O in an O.sub.2 /H.sub.2 O gas mixture unless the concentration of the other oxygen containing gas is known or its concentration remains fixed. Such prior art devices cannot be used to measure the concentration of H.sub.2 O in an O.sub.2 /H.sub.2 O gas mixture if the amount of O.sub.2 is variable because, if a voltage is applied to a pump cell sufficient to disassociate the H.sub.2 O, the O.sub.2 interferes. That is, the O.sub.2 is pumped out at a voltage lower than that required to disassociate H.sub.2 O. The concentration which could be determined in this instance would be the combined concentration of O.sub.2 +H.sub.2 O. Similarly, in a CO.sub.2 /H.sub.2 O gas mixture having variable amounts of these gases, the concentration of H.sub.2 O can not be determined using the technology of prior art devices because if a voltage is applied across the pump cell sufficient to disassociate the H.sub.2 O, the CO.sub.2 would similarly be disassociated. Thus, the concentration which could be determined in this instance would be the combined concentration of CO.sub.2 +H.sub.2 O.
It would be desirable to have a device capable of measuring the relative amount of one oxygen containing gas, e.g., CO.sub.2 or H.sub.2 O in a gas mixture comprising variable amounts of, e.g., O.sub.2, CO.sub.2, H.sub.2 O, and N.sub.2. It would further be desirable if this device had good reliability and were durable for use in automotive applications, i.e., in a high temperature environment, not be complex and be reasonable in cost. The present invention describes a method and device for measuring the relative amount of an oxygen containing gas in a mixture of "interfering" oxygen containing gases which overcomes the deficiencies of prior art devices and is well suited for use as an automotive humidity sensor.
Embodiments of the device of the present invention are similar to the two cell devices in that they comprise two solid electrochemical oxygen pump cells, which may define a cavity between them or be similar to the structure of the Logothetis et al patent discussed above. The present invention device, however, does not use one cell for oxygen-pumping and the second for oxygen-sensing as in the two cell devices described above. Rather, the invention claimed herein uses both cells as O.sub.2 -pumping cells.
In commonly assigned U.S. application Ser. No. 55,821 to Logothetis et al filed May 29, 1987 and entitled "Exhaust Gas Recirculation Sensor and Method", a device and method is disclosed for measuring the relative amount of exhaust gas recirculation (EGR) in a combined intake air and exhaust gas mixture of an internal combustion engine (termed the "ambient atmosphere"). This invention relates to a device which uses both cells as O.sub.2 -pumping cells. In one embodiment of that device, two oxygen pump cells are arranged so that, with them, a restricted volume is defined, the volume being linked to the ambient atmosphere through an aperture. A first external circuit means is coupled across the first pump cell to apply a constant voltage across the first cell so that all oxygen molecules are pumped out of the volume. A second external circuit means is coupled to the second pump cell to apply a constant voltage across the second pump cell so that all CO.sub.2 and H.sub.2 O molecules in the volume are disassociated. A third external circuit means is coupled to the second pump cell to measure a current flowing through the second pump cell. Since the current flowing through the second pump cell is directly related to the amount of (CO.sub.2 +H.sub.2 O) molecules which are disassociated, the current is proportional to the relative amount of CO.sub.2 plus H.sub.2 O in the ambient atmosphere. For engines controlled at the stoichiometric air-fuel ratio, the relative amounts of (CO.sub.2 +H.sub.2 O) in the ambient atmosphere can be related to the amount of EGR. Thus, knowing the relative amount of (CO.sub.2 +H.sub.2 O) in the ambient atmosphere by means of that device, the EGR can be determined.
The device disclosed herein comprises similar structure configuration and circuitry as in U.S. application Ser. No. 55,821 described above. That application, however, does not disclose a method or device for measuring the relative amount of only one oxygen containing gas, e.g., only the CO.sub.2 or only the H.sub.2 O as is Possible according to the present invention.
As described above, the device of U.S. application Ser. No. 55,821 determines the relative amount of the combination of two oxygen containing gases, in particular, CO.sub.2 +H.sub.2 O, in a gas mixture further comprising oxygen. This can also be done by the present invention. Advantageously, however, according to embodiments of the present invention the relative amount of a single oxygen containing gas in a gas mixture can be determined, even when the mixture contains variable amounts of each of the gases. Thus, while according to the present invention the relative amount of, e.g., (CO.sub.2 +H.sub.2 O) in an O.sub.2, CO.sub.2, and H.sub.2 O gas mixture can be determined as in the invention of '821 discussed above, advantageously the relative amount of, e.g., CO.sub.2 and the relative amount of H.sub.2 O can also be individually determined in a O.sub.2 /CO.sub.2 /H.sub.2 O gas mixture, even if the mixture contains variable amounts of each of the gases.